High performance reworkable heatsink and packaging structure with solder release layer and method of making

ABSTRACT

A method of making and a high performance reworkable heatsink and packaging structure with solder release layer are provided. A heatsink structure includes a heatsink base frame. A selected one of a heatpipe or a vapor chamber, and a plurality of parallel fins are soldered to the heatsink base frame. A solder release layer is applied to an outer surface of the heatsink base frame. The solder release layer has a lower melting temperature range than each solder used for securing the selected one of the heatpipe or the vapor chamber, and the plurality of parallel fins to the heatsink base frame. After the solder release layer is applied, the heatpipe or the vapor chamber is filled with a selected heat transfer media.

FIELD OF THE INVENTION

The present invention relates generally to the data processing field,and more particularly, relates to a high performance reworkable heatsinkand packaging structure with solder release layer and a method of makingthe high performance reworkable heatsink and packaging structure.

DESCRIPTION OF THE RELATED ART

U.S. Pat. No. 6,084,775 to Bartley et al, issued Jul. 4, 2000, andassigned to the present assignee, discloses heatsink and packagestructures with a fusible release layer. Aluminum heatsinks are platedwith a solderable layer and are overplated with a solder release layer.The release layer comprises a tin-lead-indium alloy. The heatsinks aremounted on individual IC modules or banks of IC modules that areinterconnected to a printed circuit card. A mechanically compliant,thermally conductive adhesive is used to join the heatsinks to themodules. An oxide formed on the release layer readily bonds with thethermally conductive adhesive. In the event that heatsinks need to beremoved to repair or rework the modules, local heat may be applied tomelt the release layer to remove a heatsink without need for use ofsignificant applied torque and normal forces. Because the release layerhas a low melting point that affords easy separation from the adhesivelayer, both component delaminations and the partial reflow or melting ofsolder joints on adjacent components are eliminated from the heatsinkremoval process.

Today high performance semiconductor modules have increased demands forthe cooling of the semiconductor die in the module. As a result, exotic,complicated and thus expensive thermal solutions are being implementedin the industry.

Many of the current solutions require the use of very fragile, extremelythin thermal interface material bondlines that thermally couple complexcooling devices, such as heatsinks possessing vapor chambers or integralheatpipes to the electronic component to maximize cooling efficiency.

One major problem with the current solutions is that the thermalinterface between cooling device and electronic component is very thinand very weak. Because of this thin bondline geometry and the lack ofintrinsic strength of high performance bondline materials, very highstresses on the thermal interfaces can result and make these materialsprone to material movement, which ultimately leads to thermaldegradation and device failure.

To circumvent these issues high bond strength, thin bondline thermalinterface adhesives can be used but their bonds are typically permanentand due to lack of reworkability, do not allow for manufacturabilitypackaging configuration to be established.

It is therefore very desirable to create a packaging structure, whichfacilitates creation of a high performance high reliable thermalinterface structure that is also readily separable in order tofacilitate module and circuit board repair.

SUMMARY OF THE INVENTION

Principal aspects of the present invention are to provide a highperformance reworkable heatsink and packaging structure with solderrelease layer and method of making the structure. Other importantaspects of the present invention are to provide such high performancereworkable heatsink and packaging structure with solder release layerand method of making the structure substantially without negative effectand that overcome many of the disadvantages of prior art arrangements.

In brief, a method of making and a high performance reworkable heatsinkand packaging structure with solder release layer are provided. Aheatsink structure includes a heatsink base frame. A selected one of aheatpipe or a vapor chamber, and a plurality of parallel fins aresoldered to the heatsink base frame. A solder release layer is appliedon an outer surface of the heatsink base frame. The solder release layerhas a lower melting temperature range than each solder used for securingthe selected one of the heatpipe or the vapor chamber, and the pluralityof parallel fins to the heatsink base frame. After the solder releaselayer is applied, the heatpipe or the vapor chamber is filled with aselected heat transfer media.

In accordance with features of the invention, solder release layer has alower melting temperature range than other higher melting point soldersused for attaching the heatpipe or the vapor chamber, and the pluralityof parallel fins to the heatsink base frame. The higher melting pointsolders include, for example, a selected one of Sn—Pb alloys or Pb-Freealloy compositions typically used for surface mount technology (SMT)assembly such as Sn-rich Sn—Cu—Ag solder compositions. The solderrelease layer is a relatively thin layer, and is applied by a selectedone of solder fountain, wave solder, or selective solder platingoperations. The vapor chamber and the heatpipe are copper brazed units,optionally including Nickel plating. The heatsink base frame and finsare formed of copper. The heatsink structure is attached to the modulestructure using a high performance high adhesive bond strength thermalinterface material (TIM) applied between the solder release layer andthe module structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIG. 1 is a schematic and sectional side view not to scale of anexemplary heatsink structure including a vapor chamber in accordancewith one preferred embodiment;

FIG. 2 is a schematic and sectional side view not to scale of anexemplary high performance reworkable heatsink and packaging structureincluding a heatpipe in accordance with another preferred embodiment;

FIG. 3 is a schematic and sectional side view not to scale of anexemplary high performance reworkable heatsink and packaging structureincluding the high performance reworkable heatsink structure of FIG. 1in accordance with one preferred embodiment;

FIG. 4 is a schematic and sectional side view not to scale illustratinglocal heat application to the high performance reworkable heatsink andpackaging structure of FIG. 2 or FIG. 3 in accordance with one preferredembodiment; and

FIG. 5 is a flow diagram illustrating exemplary steps for making thehigh performance reworkable a high performance reworkable heatsink andpackaging structure of FIG. 3 including the heatsink structure of FIG. 1in accordance with the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with features of the preferred embodiments, a solderrelease layer is created on a vapor chamber or heatpipe heatsinkstructure. This release layer interface material must be compatible withall fabrication requirements and attach operations incorporated into theheatsink structures and requires selection of a release solder materialwith suitable melting hierarchy relative to the chamber, heatpipe andfin attach operations used to create the heatsink component and also hasadequate working range in solid state to afford reliable interfaceformation with the electronic component to which the heatsink structureis attached and compatibility with the balance of interconnects presentthat are used to affix the module to the circuit board as well.

In accordance with features of the preferred embodiments, after creationof the heatsink structure the heatsink is placed and attached onto anelectronic module device surface using a high performance high adhesivebond strength thermal interface material (TIM) present between thesolder release layer on the bottom of the heatsink and the module/die.If removal of the heatsink is desired, the assembly is heatedsufficiently in order to drive melting of the solder release layer onthe heatsink to allow removal of the heatsink from the adhesive TIMbondline. An exemplary preferred vapor chamber heatsink structure isillustrated in FIG. 1. Also as shown in FIG. 2 an exemplary heatsink andpackaging structure includes a heatpipe heatsink structure mechanicallyaffixed using a Non-Influencing Fastener (NIF) hardware configuration.Also as shown in FIG. 3 an exemplary heatsink and packaging structureincludes the vapor chamber heatsink structure of FIG. 1.

Having reference now to the drawings, in FIG. 1, there is shown anexemplary heatsink structure generally designated by the referencecharacter 100 in accordance with one preferred embodiment. Heatsinkstructure 100 includes a heatsink base frame 104, a vapor chamber 106 ofthe preferred embodiment, and a plurality of parallel fins 108. A solderrelease layer 110 is formed on an outer or lower surface of the heatsinkstructure 100.

The solder release layer 110 of the high performance heatsink structure100 enables removal of the heatsink structure or enables reworkability.The solder release layer 110 is applied after first building theheatsink base frame 104 and attaching the vapor chamber 106 and the fins108 to the heatsink base frame 104. The heatsink base frame 104, thevapor chamber 106 and the fins 108 typically are formed of copper. TheCu brazed vapor chamber 106 optionally includes Ni plating. A Ni platedAl frame can be used for the heatsink base frame 104.

First attaching steps include soldering the vapor chamber 106 to theheatsink base frame 104 using a selected solder 114 and soldering thefins 108 to the heatsink base frame 104 and top surface of vapor chamber106 using a selected solder 112. This is accomplished using either asingle solder attached operation or a hierarchical two step solderingoperation by using higher melting point solders such as Sn—Pb alloys orPb-Free alloy compositions typically used for surface mount technology(SMT) assembly such as Sn-rich Sn—Cu—Ag solder compositions.

Referring now to FIG. 2, there is shown an exemplary high performancereworkable heatsink and packaging structure generally designated by thereference character 200 in accordance with another preferred embodiment.High performance reworkable heatsink and packaging structure 200includes a heatsink structure generally designated by 202.

Heatsink structure 202 includes a heatsink base frame 204, a heatpipe206, and a plurality of parallel fins 208. A low melting point solderrelease layer 210 is formed on a lower surface of the heatsink baseframe 204. Similar operations also are used in the construction of theheatsink structures 202 possessing heatpipes 206 instead of vaporchambers 106 of the high performance heatsink structure 100. The fins208 are soldered to the heatsink base frame 204 using a selected solder212 and the heat pipes 206 are soldered into the heatsink base frame 204using a selected solder 214. The heatsink base frame 204, the heatpipe206 and the fins 208 typically are formed of copper. The Cu brazedheatpipe 206 optionally includes Ni plating. The heatsink base frame 204can be copper or Ni plated Al.

In accordance with features of the preferred embodiments, after the bulkheatsinks 100 and 202 are made and possess embedded, soldered in vaporchambers 104 or heatpipes 206, and also have solder attached fins 108,208 in position, the low melting point solder release layer 110, 210 isapplied to all, or a portion of the heatsink base frame 104, 204. Thisrelease layer 110, 210 is typically very thin, for example less than 10microns, and can be applied in a number of ways including solderfountain, wave solder, or selective solder plating operations. In someinstances it may also be desirable to have the heatsink base frame 104,204 preplated with a barrier material such as Ni to prevent long termelevated temperature base metal-to-solder release layer interdiffusion.

Various solder release layer candidate materials including varioussuitable alloy candidates having melting ranges in the general vicinityof 117-125° C. form the solder release layer 110, 210, includingmultiple alloys manufactured and sold by Indium Corporation of America,and designated by Indalloy numbers. For example, an Indalloy number 1including 50/50 wt % Sn—In alloy and a Melting range of 118-125° C. canform the solder release layer 110, 210. Another example, an Indalloynumber 67 including 58/42 wt % Bi—Pb alloy and a melting range (MR) of124-126° C. can form the solder release layer 110, 210. Other suitablealloy candidates for the solder release layer 110, 210 having a setmelting range in a selected temperature range between 117-133° C.include:

Indalloy numbers 13 a 70/15/9.6/5.4 wt % In—Sn—Pb—Cd alloy MR 125° C.;

Indalloy numbers 62 a 55/44/1 wt % Bi—Pb—Sn alloy MR117-126° C.;

Indalloy numbers 64 a 55/44/1 wt % Bi—Pb—Sn alloy MR120-121° C.;

Indalloy numbers 70 a 40/40/20 wt % In—Sn—Pb alloy MR121-130° C.;

Indalloy numbers 71 a 52/48 wt % Sn—In alloy MR118-131° C.;

Indalloy numbers 73 a 58.84/41.16/2 wt % Bi—Sn—Pb alloy MR128-133° C.;

Indalloy numbers 255 a 55.5/44.5 wt % Bi—Pb alloy MR124° C.;

Indalloy numbers 1E a 52/48 wt % In—Sn alloy MR118° C.

After the solder release layer 110, 112 is applied, the vapor chamber106 of the heatsink structure 100 and heatpipe 206 of the heatsinkstructure 202 are then filled with suitable heat transfer media,typically water and are then sealed, for example, using a local brazingoperation.

In accordance with features of the preferred embodiments, the heatsinkstructures 100, 202 are completed and then adhesively affixed to modulesurfaces using a high performance high adhesive bond strength thermalinterface material (TIM). If additional strain relief is required tosupport the heatsink and mechanical hardware surrounding the packagingit is added thereafter, rendering for example respective structures 200,300 as shown in FIGS. 2 and 3.

High performance reworkable heatsink and packaging structure 200includes an associated module structure 220 including one or moreelectronic module devices 222 carried by one or more modules 224 mountedto a printed circuit board 226, for example, by a plurality ofconnection pins, such as solder columns 228. The heatsink structure 202is attached to the chip surface of module devices 222 using a highperformance high adhesive bond strength thermal interface material (TIM)230 present between the bottom of the heatsink structure 202 and themodule device or chip die 222. A load frame arrangement generallydesignated 232 located between the printed circuit board 226 and theheatsink structure 202 positions and retains the heatsink structure. Aplurality of screws 234 fasten the load frame arrangement 232 and astiffener member 236 to the printed circuit board 226. The heatpipeheatsink structure 202 is mechanically affixed to the load framearrangement 232 using a Non-Influencing Fastener (NIF) hardwareconfiguration of a plurality of fasteners 238.

Referring now to FIG. 3, there is shown an exemplary high performancereworkable heatsink and packaging structure generally designated by thereference character 300 in accordance with one preferred embodiment.High performance reworkable heatsink and packaging structure 300includes the high performance reworkable heatsink structure 100 of FIG.1 and an associated module structure 302. High performance reworkableheatsink and packaging structure 300 includes one or more electronicmodule devices 322 carried by one or more modules 324 mounted to aprinted circuit board 326, for example, by a ball grid array (BGA) 328.The heatsink structure 100 is attached to the chip surface of moduledevices 322 using a high performance high adhesive bond strength thermalinterface material (TIM) 330 present between the bottom of the heatsinkstructure 100 and the module device or chip die 322. A mechanicalfastening arrangement generally designated 332 is located between theprinted circuit board 326 and consists of a plurality of alignment posts334 affixed to the heatsink base frame 104, extending through theprinted circuit board 326, and attached to a stiffener member 336.

FIG. 4 schematically illustrates local heat application generallydesignated by the reference character 400 with a high performancereworkable heatsink 100 and module structure 302. Local heat application400 is indicated by a plurality of arrows respectively labeled A and Bfor heatsink removal when required, for example, to facilitate eitherreapplication or module removal from the board. The assembly is heatedto a temperature above which the solder release layer 110 melts, suchthat debonding of the high performance high adhesive bond strength TIM330 can occur.

Referring now to FIG. 5, there are shown exemplary steps for making thehigh performance reworkable heatsink and packaging structure, forexample, structures 200, 300 as shown in FIGS. 2 and 3 in accordancewith the preferred embodiments. It should be understood that forsimplicity the vapor chamber heatsink structure 100 only is described,while the same fabrication method equally applies to the heatpipeheatsink structure 204.

As indicated in a block 500, first copper Heatsink (HS) base frame 104is machined to facilitate attach of copper vapor chamber 106 and copperfins 108. Next the step is to fixture unfilled vapor chamber 106 into HSbase frame 104 and fins 108 onto the copper HS base frame 104 and vaporchamber 106 as indicated in a block 502. Then a next step is to applyflux or solder to attach as indicated in a block 504. The attach step atblock 504 can be a 1 or 2 step operation. The vapor chamber 106 can beformed of copper, or nickel plated copper.

The single step operation to solder fins 108 and vapor chamber 106 tobase frame 104 includes mounting fins 108 and chamber 106 simultaneouslyto base using solders with melting characteristics ranging fromapproximately 180-260 C. Solder 112, 114 used to simultaneously attachfins 108 and chamber 106 can be a variety of alloys such as:

a. 70/30 wt % Sn—Pb alloy having a melting range 183-257° C.;

b. eutectic Sn—Pb (63-37%) MP 183° C., or 60/40 Sn—Pb Solder having amelting range approximately 183-190° C.;

c. Pb-Free Alloy such as 95% Sn, 4% Ag 1% Cu having a melting rangeapproximately 217-225° C.;

d. Pb-Free Alloy such as 89% Sn, 8% Zn, 3% Bi having a melting rangeapproximately 190-197° C.

The two step operation requires solder melting hierarchy where first tomount or solder the vapor chamber 106 into the HS base frame 104 usesthe solder (a.) 70/30 wt % Sn—Pb, (melting range 183-257° C.); or forPb-Free Attach use solder (c.) Pb-Free Alloy such as 95% Sn, 4% Ag 1% Cuhaving the melting range approximately 217-225° C. Second the fins 108are attached to base frame 104 after the vapor chamber 106 has beenattached to base with the fin attach using solder (b.) eutectic Sn—Pb(63-37%) MP 183° C., or 60/40 Sn—Pb Solder having a melting rangeapproximately 183-190° C.; or for Pb-Free Attach use solder (d.) Pb-FreeAlloy such as 89% Sn, 8% Zn, 3% Bi having a melting range approximately190-197° C.

A next optional step is to machine flat the base of heatsink vaporchamber 106, if necessary if the vapor chamber base 106 is notsufficiently flat as indicated in a block 506.

Then the thin solder release layer 110 is applied to vapor chamber baseor outer surface of heatsink structure 100 as indicated in a block 508.The thin solder release layer 110 is, for example, a 50/50 wt % Sn—Inalloy with a melting range of 118-125 C. The thin solder release layer110 can be applied by fixturing heatsink structures 100 onto a wavesolder machine equipped with an air knife to blow off excess solder orcan be applied with a solder pot or fountain and squeegee for removal ofbulk materials to provide the thin solder release layer coating.

As indicated in a block 510, then the vapor chamber 106 is filled withappropriate volume of thermal conduction fluid, typically water. Thenthe vapor chamber fill port is sealed as indicated in a block 512, forexample, crimped and soldered or sealed using local heating and localsolder application using a selected solder, where any of above examplesolders will work. This completes the vapor chamber heatsink structure100. Then the vapor chamber heatsink structure 100 is attached to themodule structure 302 to complete the high performance reworkableheatsink and packaging structure 300 as indicated in a block 514.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1-11. (canceled)
 12. A method of making a high performance reworkableheatsink and packaging structure comprising the steps of: forming aheatsink structure for attachment to a module structure; forming saidheatsink structure including providing a heatsink base frame; solderinga selected one of a heatpipe or a vapor chamber to said heatsink baseframe, soldering a plurality of parallel fins to said heatsink baseframe; and applying a solder release layer on at least a portion of anouter surface of said heatsink structure; said solder release layerhaving a lower melting temperature range than each solder used forattaching said selected one of said heatpipe or said vapor chamber, andsaid plurality of parallel fins to said heatsink base frame; andattaching said heatsink structure to said module structure using a highperformance high adhesive bond strength thermal interface material (TIM)applied between said solder release layer and said module structure. 13.A method of making a high performance reworkable heatsink and packagingstructure as recited in claim 12 includes the steps of filling saidselected one of a heatpipe or a vapor chamber with a predefined volumeof a thermal conduction fluid after said solder release layer isapplied.
 14. A method of making a high performance reworkable heatsinkand packaging structure as recited in claim 12 wherein applying saidsolder release layer includes selecting a solder alloy having a selectedmelting range between 117-133° C.
 15. A method of making a highperformance reworkable heatsink and packaging structure as recited inclaim 12 wherein the steps of soldering said selected one of a heatpipeor a vapor chamber to said heatsink base frame and soldering saidplurality of parallel fins to said heatsink base frame includesselecting a solder alloy having a selected melting range between183-257° C.
 16. A method of making a high performance reworkableheatsink and packaging structure as recited in claim 15 includesselecting said solder alloy from one of: a. 70/30 wt % Sn—Pb alloyhaving a melting range 183-257° C.; b. eutectic Sn—Pb (63-37%) meltingpoint approximately 183° C., or 60/40 Sn—Pb solder alloy having amelting range approximately 183-190° C.; c. Pb-Free alloy of 95% Sn, 4%Ag 1% Cu having a melting range approximately 217-225° C.; and d.Pb-Free Alloy such as 89% Sn, 8% Zn, 3% Bi having a melting rangeapproximately 190-197° C.
 17. A method of making a high performancereworkable heatsink and packaging structure as recited in claim 15includes selecting a first one of said solder alloys for soldering saidselected one of a heatpipe or a vapor chamber to said heatsink baseframe; and selecting a second one said solder alloys for soldering saidplurality of parallel fins to said heatsink base frame.
 18. A method ofmaking a high performance reworkable heatsink and packaging structure asrecited in claim 12 wherein applying said solder release layer includesapplying a thin solder release layer having a thickness less than 10microns.